There are two main configurations of passive attenuators that are better suited to different applications, although each of these also has several variations.
- Rectangular Attenuators (straight or bend configuration) typically consist of several absorptive elements (splitters) that subdivide an aperture or duct into modules consisting of an airway with acoustically absorptive sides.
- Circular and O’Square Attenuators consist of a circular duct with an acoustically absorptive inner surface, possibly with a concentric central pod that is also acoustically absorptive.
The principles of how the acoustically absorptive splitters, linings and pods work is the same for rectangular and circular attenuators. Thicker absorptive elements (typically 300mm to 600mm) are necessary for improved low frequency attenuation. Narrower airways (typically 50mm to 100mm) provide better high frequency performance. However, the aerodynamic resistance of an attenuator is controlled by the free area proportion (airway:splitter ratio) and the velocity of air in the attenuator. This means that an attenuator with thick splitters and small airways will impose much more resistance to airflow than a similar sized one with thinner splitters and larger airways, so a much larger attenuator may be necessary.
